1. Field of the Invention
The present invention relates to light transmitting fibers such as low melting temperature fibers fabricated from polymers (e.g., polymethyl methacrylate, PMMA, etc.) which have been widely used in light transmission and illumination for industrial and medical applications. More particularly, the present invention relates to the power handling capabilities of such light transmitting fibers.
2. Description of the Background Art
The following description of the background art represents the inventors"" knowledge and not necessarily knowledge of those within the art.
Low melting temperature fibers fabricated from polymers have advantages of high numerical apertures, mechanical flexibility, and low cost. Light is normally coupled directly into these fibers from a focused light source or from another fiber or fiber bundle by proximity coupling (xe2x80x9cbutt couplingxe2x80x9d).
Due to the low melting temperature of such polymers and their light absorbing properties, the power handling capability of a polymer fiber is relatively low. A critical parameter that restricts the total amount of light capable of being transmitted is the increase in temperature resulting from absorption within the fiber which, though negligibly small, causes xe2x80x9cthermal run-awayxe2x80x9d and melting of the polymer fiber. The causes for the endface of the fiber gradually melting include not only light absorption but also non-transmitted light which is converted into heat as the result of mismatches in size and numerical aperture between the light source and the polymer fiber.
Light absorption is local power intensityxe2x80x94dependent. As a result, the shape of the intensity profile, which is typically Gaussian, is a major determinating factor for the input power at which a polymer fiber melts. In addition, if appropriate light-filtering is not employed to remove UV and IR light, the fibers will discolor and degrade in their transmission abilities. The latter is accelerated by high light intensity and heat generated from non-transmitted light.
Improvements can be achieved by adding a spatial filter between the light source and the polymer fiber. The spatial filter can consist of a heatsink and an aperture placed at the input interface of the polymer fiber and light source, or of a short piece of glass or quartz fiber, having a numerical aperture equal to or smaller than that of the polymer fiber and a length sufficient to remove unguided modes of light. The latter improves the performance in power transmission of a polymer fiber from the 100 mW range to the range of 300 to 400 mW for a 1-mm diameter PMMA fiber, for example. Nevertheless, the low melting temperature of such polymer fibers severely restricts the maximum power able to be coupled into a single polymer fiber or fiber bundlexe2x80x94as compared with a similar glass or quartz core fiber. For larger diameter single fiber typically 3 mm or larger, it is known in the art to interpose a glass fiber bundle between the light source and the polymer fiber. U.S. Pat. No. 4,986,622 (Martinez) teaches the use of a glass fiber bundle to transmit light from a light source to a bundle of plastic polymer fibers. Although it is known that such spatial filters increase the light able to be transmitted through a plastic fiber without degradation, the degree of spatial filtering is limited to eliminating unguided modes without modifying the shape of the Gaussian profile.
Although spatial filtering eliminates unguided modes, the input light has a Gaussian-like intensity profile for which the peak power is highest in the center and lowest at the perimeter of the beam. As a result, the intensity at the center of the end face of the polymer fiber becomes the limiting factor in the power handling capability of the fiber since high peak power above the absorption threshold will cause degradation of the fiber.
The present invention, increases the power handling capability of polymer fibers by broadening or modifying the Gaussian intensity profile without significantly decreasing the efficiency of coupling light into a polymer fiber. In the preferred embodiment, the overall intensity profile is more uniform and the peak intensity is lower. This allows more total power to be coupled into the polymer fiber without melting the input interface of the polymer fiber. The methods disclosed facilitate significant improvements in power transmissionxe2x80x94enabling, for example, power in excess of 1 watt to be transmitted through a 1-mm diameter PMMA fiber.
The present invention provides a new approach involving a device which modifies the light intensity profile so as to raise the threshold input power at which absorption causes thermal run-away. The device broadens the output intensity profile of the light source before it is coupled into the single polymer fiber or fiber bundle. By broadening the intensity profile, the energy at the center of the Gaussian intensity profile is distributed to the perimeterxe2x80x94thereby reducing the peak power intensity. Since absorption depends on the energy per unit area, a decrease in absorbed energy reduces the probability of reaching the temperature threshold at which the polymer will begin to melt. Hence, the accumulated heat is decreased over that which occurs if the Gaussian profile is not modified, thus ensuring the survival of the fiber at a much higher input power. This in turn increases the maximum power that the polymer fibers can transmit without thermal damage.
According to a first aspect of the invention, a method for increasing the power handling capability of polymer fibers, includes the steps of: a) emitting light, filtered to remove wavelengths below about 400 nm and above about 700 nm, and having a Gaussian intensity profile from a fiber light source; b) broadening the Gaussian intensity profile so that energy at the center of the Gaussian intensity profile is distributed to the perimeter of the profile to reduce the peak power intensity of the emitted light before it is launched into the polymer fiber; c) transmitting the emitted light into at least one polymer fiber.
According to a further aspect of the invention, the Gaussian intensity profile is kept such that the total energy launched (i.e., transmitted) into the at least one polymer fiber is substantially unchanged, but the peak power intensity is diminished.
According to another aspect of the invention, the step of broadening the Gaussian intensity profile includes providing a diffuser between the emitted light and the at least one polymer fiberxe2x80x94the diffuser is most preferably a fused fiber bundle.
According to another aspect of the invention, the end face of the fused fiber bundle is at an angle.
A preferred functional attribute of such a broadening device is that for a given power level from a source of light; for example, light from a fiber optic or from an arc lamp, the total energy impinging on the face of the polymer fiber must be unchanged, but the peak power must be diminished. In effect, the transformation that must be effected is one of dispersing light from the center, low angle light, to the perimeter, high angle light, thereby decreasing the local power density at each point in the center of the Gaussian profile.
The above and other advantages, features and aspects of the invention will be more readily perceived from the following description of the preferred embodiments thereof taken together with the accompanying drawings and claims.